Single doOnSuccess / doOnError

[JolandaVerhoef]

Observable has doOn* methods to put side effects in. Single has no such methods yet.

[akarnokd]

Single is in experimental state. If you wish, you can post a PR that adds these methods.

[hyleung]

@JolandaVerhoef I believe #3417 and #3419 (courtesy of @artem-zinnatullin) add doOnSuccess and doOnError, respectively.